1. Field of the Invention
The present invention relates to equatorial Medium Earth Orbit (MEO) satellite constellation hand-off and diversity-combining by earth-station antenna receivers.
2. Background of the Related Art
The O3b Satellite Constellation consists of 8 equatorial orbit satellites (soon expanding to 16 satellites, then 24 satellites) owned by O3b Networks, Ltd, and deployed at a MEO altitude of about 8063 km. Relative to a fixed point on earth, all satellites trace an identical arc across the sky; customers and Teleports use antennas to track the satellites and maintain network connectivity. O3b Satellites serve terrestrial regions within ±45 degrees latitude using steered antennas.
Due to the satellites' orbital spacing, there is a period when two satellites are both visible to both the Teleport and its Customers (“dual satellite access period”). A portion of the dual satellite access period is a pre-planned “hand-off duration”.
Customer Terminals are in contact with at least one satellite. Maintaining contact requires a two antenna configuration: one (or both) for link use in the inter-hand-off interval, the other, during hand-off, pre-positioned to acquire an ascending satellite.
O3b has described using switching off-the-shelf modems' outputs to achieve hand-off using make-before-break and break-before-make strategies and for managing the resulting duplication or elimination of packets by higher levels in the protocol stack.
To protect the integrity of data, TCP packets have sequence numbers, timestamps, flow-control, congestion control and checksums. During TCP exchanges, ACK and NACK packet types inform the sender whether packets were received. Clearly, in the break-before-make approach, packet re-transmission becomes necessary. This causes the conventional congestion control process that uses ACKs and NACKs vis-a-vis timers to throttle back data fed to the network and increasing latency. A make-before-break approach can ensure that packets are not lost due to switching. However, it is desirable to perform the hand-off at the physical layer in order to eliminate duplication of equipment, overhead and latency.